The present invention relates to an image processing apparatus and method which forms an image by emitting a laser beam in accordance with image signals and, more particularly, to processing which stabilizes the quantity of light emitted by the laser beam.
There are image forming apparatuses adopting a variety of printing methods, such as electrophotographic, ink-jet, thermosensible-transfer and wire dot, which form an image in accordance with image signals sent from a host computer.
Among the above methods, presently, the electrophotographic printing method has been widely used because of its features of high-speed operation, high quality image reproduction and quiet operation. Particularly, since it is possible for an image forming apparatus using a laser beam exposure method to expose, for a period of time less than that corresponding to one dot width, by modulating the duration of a laser beam, it is possible to easily express tones by using multi-valued data for each dot. This is an advantageous feature of the electrophotographic printing method compared to the other printing methods mentioned above. This feature has become more important as color image formation is more frequently performed.
In order to form a multi-valued image, a modulator for dividing a laser beam pulsewidth into a time period corresponding to less than one dot and a formatter for converting digital image data into a laser driving signal are required.
FIGS. 1A and 1B are block diagrams illustrating examples of configurations of a printer controller.
There are two ways of setting a modulator 102. In one configuration, the modulator 102 is provided upstream from a laser driver 103 as shown in FIG. 1A, and in the other configuration, the modulator 102 is provided inside of an interface controller 101' as shown in FIG. 1B. In either case, the modulator 102 converts digital image signals (parallel signals of generally 4 to 8 bits) outputted from a formatter 110 into a serial signal, and inputs the serial signal into the laser driver 103 which drives a semiconductor laser diode (LD) 104.
Image data is inputted to the interface controllers 101 and 101' from various kinds of host computers. Therefore, the interface controller and the formatter must be able to properly process input data from various kinds of host computers. Accordingly, the formatter 110 has to be designed in conformance with the type of image data inputted from a host computer.
Therefore, in order to interface a printer engine to a variety of host computers, it is common to manufacture a printer engine which is partially completed, then at a later time attach the interface controller 101 or 101' to conform to the host computer.
Further, there is a demand to increase the degree of freedom of the interface controllers 101 and 101'in forming an image, to the extent possible. For example, the interface controller 101', shown in FIG. 1B, which generates digital image signals as well as a serial signal for driving the LD 104, is superior to the interface controller 101 shown in FIG. 1A, which does not generate a serial signal.
The degree of freedom of a controller in forming an image is called "the degree of freedom for designing". More specifically, the interface controller 101 in the configuration shown in FIG. 1A can deal with tone data represented by digital signal values only. By contrast, the interface controller 101' in the configuration shown in FIG. 1B can convert digital signals (parallel signals) into an analog signal by using the modulator 102. Accordingly, the interface controller 101' can completely control how long the laser beam is to be emitted (emission time). This is known as a "high degree of freedom for designing".
However, there is a problem in outputting a half-tone image in a stable state in the configuration shown in FIG. 1B. The laser beam pulsewidth varies since there are variations in pulsewidth of a modulated signal (serial signal) for driving the LD 104 and, even though a signal of a fixed pulsewidth is inputted to the laser driver 103, since the driving threshold and current-output light (I-L) characteristics of a laser diode used as the LD 104 in the configuration shown in FIG. 1B varies one from another. For example, in order to print an image in a resolution of 600 dpi, which is a typical resolution, by a laser beam printer, a laser beam pulse whose width is about 40 ns per pixel has to be divided into a plurality of laser beam pulses, while the deviation in pulsewidth is a few nano-seconds.
In the configuration shown in FIG. 1A, the relationship between the multi-valued digital image data and the laser beam pulsewidth can be adjusted for each printer engine in advance. However, it is time consuming and costly to adjust the relationship for each printer engine to print a half-tone image faithfully and in stable state in accordance with image data outputted from the formatter 110.
Further, the following problems are posed in the aforesaid techniques.
Namely, although an interface controller for a laser beam printer having the configuration shown in FIG. 1B is advantageous with respect to the degree of freedom attained in image formation, it is rarely used because of the aforesaid reason, and if it were used, the half-tone output would not be stable.
In order to compensate for the aforesaid defect, there is a method of forming test patches on an electrostatic drum, measuring their densities, and correcting image data on the basis of the measured densities. However, since the measured density values in this correcting method include variations due to such factors as the developing unit and the electrostatic drum, the laser beam pulsewidth is not properly corrected. In practice, in a case where the density decreases are due to deterioration of the developing unit, if every laser beam pulsewidth were made longer, it would not be possible to obtain the maximum density. Moreover, the tone of the output image would be abnormal.
Even if characteristics of a developing unit and an electrostatic drum are corrected by adjusting developing bias and charged potential in accordance with the measured densities of the test patches, there are nonetheless large variations in laser beam pulsewidths. Accordingly there is a dilemma in that neither of the half-tone test patches can be a reference for making corrective adjustments.